Alexandra Palace is not a mosque, but last week it looked like one. Set designers had gone to work, painting domes and minarets, so that the interior of the building where British television began looked more like north Damascus than north London, more Mecca than Muswell Hill. The change was for Islam Expo, a four-day festival of debate, music and culture that brought in tens of thousands of people, instantly becoming the biggest event of its kind in the history of Britain's Muslim community.

I was at a panel on the origins of terrorism: "religious or political?" The star turn was Palestinian-born Dr Azzam Tamimi of the Muslim Association of Britain (MAB). Asked whether Hamas should recognise Israel and renounce violence, he launched into a full-throttle rhetorical assault, insisting: "It is the same as asking rape victims to recognise that their rape was legitimate ... We will never do that, never!"

While the audience were loudly applauding and cheering, I was struck by two related thoughts, both of which have become especially pressing in the globalised world of the 21st century - in which it is now common, rather than unusual, for people to live far away from the lands with which they strongly identify.

For this is the age of the diaspora. There are Indians in America, Turks in Germany, Algerians in France, and everybody in London. Many of these communities stay passionately connected to the politics of the old country. But their view of that politics is different from those who are living it, day to day. Thus is born the phenomenon of the irresponsible diaspora.

I saw it up close in the US, when I met Irish-Americans whose nostalgic brand of Irish nationalism made them much more stubbornly hardline than the leadership of Sinn Féin, those who lived the reality of contemporary life on the Falls Road. As one senior republican put it to me, these third-generation Americans in Boston and New York were "more Catholic than the Pope".

The phrase doesn't fit Dr Tamimi, but the sentiment does. For most Palestinians on the ground had to give up the luxury of such dogmatism long ago. Even Hamas, which stands alongside the MAB in the worldwide fraternity of the Muslim Brotherhood, is quite clearly on its way to reaching an acceptance of Israel's existence within the 1967 borders. It has taken tentative steps in that direction already, most recently in its backing for the so-called "prisoners' document" that implies acceptance of Israel. For the sake of trying to end the distress they live with every day in Gaza and the West Bank, they can no longer afford the ideological purity of Dr Tamimi's "never".

Intriguingly, tension on the other side of this conflict, between Israel and the Jewish diaspora, often runs the other way. True, in the Oslo period of the 1990s the minority Zionist Organisation of America stood to the right of Yitzhak Rabin, insisting it understood Israel's security needs better than Israel itself. But a more frequent clash is of the kind seen last week, when 300 British Jews took a full-page ad in the Times to call on Israel to halt its "collective punishment" of the people of Gaza. I share the signatories' horror at the bombing of a Palestinian power station and cutting-off of fuel supplies: such actions are morally indefensible. And I reject the complaints of those Israelis who denounce their diaspora critics, saying that only those who live with the Israeli reality have any right to comment. That said, I do wonder why these concerned Jews did not place an ad in an Israeli newspaper. If their motive was moving Israeli policy, that would surely have been the right place. Instead, they have allowed their opponents to say their prime interest was advertising their own dovishness to their fellow Britons.

Still, the move has served as a timely reminder that no diaspora community, no ethnic minority, is a monolith: they are as varied as the societies with which they identify. Timely, because right now the government faces serious criticism of its view of the British Muslim community - or communities - specifically in the way they are represented.

On Friday Channel 4 will air a documentary by Martin Bright of the New Statesman in which he argues that ministers have in effect anointed as the official representatives of British Islam those who represent some of its most radical shades of opinion. Central is the Muslim Council of Britain (MCB), an umbrella body but one that is, Bright argues, sympathetic to a robust form of Islamism. Others recall the MCB's former head, Sir Iqbal Sacranie, who at the height of the Salman Rushdie affair wondered if "death is too good for him".

Bright traces the origin of this approach to the Foreign Office, which has long believed in engagement with political Islam, including the Muslim Brotherhood. Using a stream of leaked documents, Bright shows how civil servants have argued for the same policy of engagement to apply domestically. The result has been the admission to Britain of one of the Brotherhood's spiritual leaders, Sheikh Yusuf al-Qaradawi - who blesses suicide bombings against Israeli civilians, denounces homosexuality, and defends the physical disciplining of women by their husbands - on the grounds that he is in tune with "mainstream" British Muslim opinion (a move rather at odds with Tony Blair's stated belief that the Brotherhood is partly responsible for the global spread of the ideology that underpins Islamist terrorism). A succession of other, less strident Muslim leaders testify that they have been shut out of government dialogue - and denied financial help - in favour of the privileged MCB.

The key question is where, in fact, the British Muslim mainstream lies. Bright says the majority are from the more non-political Sufi tradition; others insist that young British Muslims see the MCB as, if anything, too mild and insufficiently hardline. Which is it?

Polling offers a mixed picture. Last month the Pew Global Attitudes Project found that British Muslims had far more negative views of westerners than Muslim communities elsewhere in Europe. A significant majority viewed western populations as selfish, arrogant, greedy and immoral - attitudes more prevalent in Muslim societies in the Middle East and Asia than among other European Muslims. Another poll last week further found that 13% of British Muslims believe the 7/7 bombers should be regarded as "martyrs".

Of course, the flipside of that finding is that a majority of British Muslims believe no such thing. Indeed the poll found most saying the government has failed to combat extremism - a greater proportion with that view than among the British population as a whole.

There is, then, evidence available to both sides of this argument: those who say British Muslims are radical and therefore have to be met where they are - not where others would like them to be - and those who say that this community is essentially moderate, but is regularly stirred up by hardliners who enjoy the misguided support of the government.

My own view is that the government is making a mistake if it hands the franchise of dialogue over to a single organisation or strand of opinion. It may be more convenient, but it is mistaken. For diasporas and religious communities are diverse organisms, with complex, contradictory views. They can be irresponsible and irritating, but they are never monolithic. The annoying reality is, you have to speak to all of them.